Spatial and temporal control of the far-field and near-field distribution of the THz signals is critical to enabling applications namely, high-speed wireless communication, high-resolution imaging, and sensing. Particularly, being able to manipulate THz fields at sub-wavelength scales at high speed can lead to the development of versatile THz sensing and communication systems for which rapid and reconfigurable beamforming is needed. This can also enable new forms of energy-efficient, compact, and lens-less THz sensing and imaging systems with a low number of pixels. Metasurfaces are two-dimensional surfaces with precisely designed scatterers that create controlled field transformation of incident wavefronts across the properties of amplitude, phase, frequency, and polarization. 

Engineering the fundamental material properties such as electric and magnetic susceptances and thereby the overall surface impedance is a powerful technique to manipulate electromagnetic (EM) wave propagation and metasurfaces are one of the ways of achieving the same. Unlike conventional optical or antenna design, such surfaces are designed with carefully engineered scattering structures at sub-wavelength length scales that can collectively provide abrupt phase and amplitude changes of the incident THz field. This allows us to systematically design flat metasurfaces with scattering structures capable of a desired EM transformation.  Metasurfaces in conjugation with reconfigurable high speed electronics, can allow new methodologies through this ‘material as a device’ design approach. In many applications, metasurfaces are paving the way towards low-loss, flat, and ultra-thin form factor components.

Here, we present a modular approach for programmable THz metasurface with fully integrated silicon chip tiles (See Fig. 1). Silicon chip architecture lends itself naturally for modular and repetitive architectures with seamless interconnects. In this article, we demonstrate this design methodology with silicon chip tiles fabricated in an industry-standard 65  nm  CMOS  process.   Each chip encompasses an array of 12×12 elements.   Each element is individually addressable and programmable with 8-bit control. Taking this as a unit tile, we demonstrate a 2×2 array of these tiles, creating a 2D surface with 576 meta-elements that are independently digitally reconfigurable at a maximum clock speed of 5 GHz. We demonstrate the capabilities of the surface with amplitude modulation with a switching depth of 25 dB, operation as a spatial light modulator, reconfigurable beamforming of ± 30^o with phased surfaces, and programmable holographic projections at 0.3 THz. The tile-based approach is scalable to even larger arrays and potentially to the neighboring spectral regions. Some of the illustrative results are shown in Fig. 2.

The high-speed modulation of the surface at multi-GHz speed allows the surface to establish a high data rate THz communication link. With the ability to control amplitude and phase of the transmitted wave, the surface can be operated as a back-scatter radio that can convert a continuous-wave signal into a modulated one. This could potentially lead to THz surface modulators that enable high-speed THz transmitters that circumvent the need for frequency mixers. Such systems could be of use in future THz sensing and imaging systems, and in multi-Gbps THz wireless links. These applications can enable the next-generation of ubiquitous THz connectivity with high density deployment of low cost and low power THz wireless nodes.

For more details, check out our paper “A High-Speed Programmable and Scalable Terahertz Holographic Metasurface based on Tiled CMOS Chips” on Nature Electronics, Dec 2020.